The major goal of this project is to understand how potassium ions are transported by epithelia. Two major epithelia which are responsible for the regulation of plasma potassium levels are the kidney and the colon. Studies of the rabbit descending colon are proposed as a model system for investigating K+ transport mechanisms. Specifically, the regulation of apical membrane Na+ and K+ permeabilities by a number of agents including aldosterone, cyclic nucleotides, and metabolic factors will be examined. Novel information will be obtained about basolateral membrane properties, specifically, K+ channels and their kinetic properties. Basolateral membrane K+ channels will also be assessed in toad bladder, Necturus gall bladder and locust rectal epithelium and compared to those from the colon. Regional differences in colonic K+ transport will be investigated and the mechanisms of Na+ and K+ transport in the human descending colon will be determined. The methods used to achieve these goals include: (1) conventional and ion-sensitive microelectrode techniques, (2) current-voltage relationships, (3) radio-isotopic flux determinations, (4) impedance analysis and morphometric measurement and (5) current fluctuation analysis. The information gained from these converging methods will be incorporated into an equivalent circuit model of the mammalian colon which takes into account morphological features, K+ channel properties and the regulation of K+ transport across the epithelium.